rust-matchers: better support file_set in IntersectionMatcher...
rust-matchers: better support file_set in IntersectionMatcher
Previously, if both of the input matchers had non-exact file sets,
IntersectionMatcher would simply have an empty fileset.
Now, we duplicate Python behavior: we *union* the filesets of the
matchers in that case. This makes some sense, because without exact
input file sets, we can't do an exact intersection and must
over-approximate.
Concretely, this is necessary because the file_set is the list of
files that were explicitly listed by the user, and we want to provide
explicit errors for all such files, from both matchers.
